The present invention relates to diversity receivers used in radio communication concomitant with fading.
Diversity reception has been known as means for mitigating the influence of fading in radio communication and recently, has been used widely in a radio apparatus for mobile communication such as car telephone or portable telephone in which deep fading due to movement of a station of its own or a mate station is generated.
The most general diversity receiver is of the post-detection synthesis type in which there are provided a plurality of antennas and a plurality of receiving units provided in association with individual antennas and output signals of the individual receiving units are subjected to a selection/synthesis process under a condition that, for example, output signals of higher receiving signal intensity (electric field intensity, and represented by symbol RSSI) are preferentially processed, thus producing a reception output signal. But, the post-detection synthesis type has a disadvantage that the plurality of receiving units are needed as above, with the result that the construction is complicated and increased in scale and the apparatus cost becomes high.
To eliminate the disadvantage, an antenna changeover diversity receiver has been proposed in which a single receiving unit and a plurality antennas are provided and reception is carried out by suitably switching the antennas under a suitable condition.
A conventional diversity receiver will be described with reference to the accompanying drawings. FIG. 8 is a block diagram of the conventional antenna changeover diversity receiver. In FIG. 8, one of antennas 1 and 2 is selected by an antenna changeover switch 3. A receiving unit 4 demodulates a desired signal from a high-frequency signal received through the antenna 1 or 2 and delivers a reception output signal. Antenna selecting means 15 controls the antenna changeover switch 3 when RSSI falls below a predetermined changeover threshold value to cause the antenna changeover switch 3 to transfer from one antenna now in connection (now connected to the receiving unit 4) to the other.
For the antenna changeover diversity reception, two methods called a switch and stay method (SS method) and a switch and examine method (SE method) have been known as detailed in, for example, a literature xe2x80x9cBase of Mobile Communicationxe2x80x9d compiled under the supervision of Okumura and Shinshi, issued by the Institute of Electronic Information and Communication Engineers of Japan, 1986.
FIG. 9 is a diagram for explaining the operation of the conventional diversity receiver. Changeover behavior pursuant to the SS method is illustrated at section (a) in FIG. 9 and changeover behavior pursuant to the SE method is illustrated at section (b) in FIG. 9. The operation of the conventional antenna changeover diversity receiver will be described below. In FIG. 9, RSSI for the antenna 1 is indicated by reference numeral 10 and RSSI for the antenna 2 is indicated by reference numeral 20. Especially, in the figure, thick curve represents RSSI for one antenna connected to the receiving unit 4 through the changeover switch 3 and thin curve represents RSSI for the other antenna disconnected from the receiving unit 4.
It is now assumed that the antenna 1 is in connection and RSSI changes owing to fading during reception as shown at (a) and (b) in FIG. 9. As the RSSI falls below the predetermined threshold value, a changeover signal is sent to the antenna changeover switch 3 and instead of the antenna 1, the antenna 2 is connected to the receiving unit. Consequently, the RSSI at the receiving unit 4 traces thick curve shown in FIG. 9.
If the distance between the antennas 1 and 2 is sufficient, the correlation between instantaneous values of RSSI for the antennas 1 and 2 is low and there is a high probability that while the RSSI for one antenna is very low, the RSSI for the other antenna is higher. Accordingly, the probability that the antenna for which the RSSI is higher is selected is high and a more excellent reception output signal than that of a receiver devoid of diversity can be obtained.
But an instance sometimes occurs in which RSSI immediately after a changeover happens to be lower than the threshold value. In such an event, pursuant to the SE method, the antenna changeover is again carried out, continuing until the RSSI becomes higher than the threshold value but pursuant to the SS method, the antenna changeover is not effected and reception by the antenna now in connection remains until the RSSI becomes higher than the threshold value. According to the aforementioned literature xe2x80x9cBase of Mobile Communicationxe2x80x9d, an average of signal to noise (S/N) ratios in the SE method is equal to that in the SS method if changeover noise generated concomitantly with a changeover is neglected. Accordingly, in the case of radio systems or broadcasting reception in which a continuous wave of, for example, analog FM or analog AM modulation is transmitted, the SS method is often used because the number of changeover operations is smaller.
Further, for improving the characteristics of the antenna changeover diversity receiver, methods have been proposed including one as disclosed in JP-A-57-10542, according to which the frequent changeover operations are prevented by lowering the threshold value after the antenna changeover, and including another as disclosed in JP-A-6-140971, according to which when the RSSI even after the antenna changeover is below the changeover threshold value, the connection of the preceding antenna is recovers.
In the aforementioned diversity receivers, however, the reception performance is disadvantageously degraded when the fading speed becomes very low at the time that the user of cordless telephone or portable telephone stops or the user keeping still uses the telephone.
More specifically, when RSSI even after an antenna changeover falls below the changeover threshold value in the SE method shown at (b) in FIG. 9, changeovers occur sequentially as described above. Since the change speed of the RSSI is slow, the sequential changeover continues for a long time, Consequently, changeover noise takes place in a demodulated signal for a long time and the communication quality is degraded. In the case of transmission of digital data, a bit error due to changeover noise can be corrected by using a correction code.
In the digital transmission, however, sections generally called frames each being a unit of several of tens of bits or several of hundreds of bits are transmitted and the correctable number of bits is limited to several bits or several of tens of bits or less and the ability to correct the bit error is exceeded when the changeover occurs frequently.
On the other hand, in the case of the SS method, the frequent changeover can be avoided but when the RSSI rather lowers after the antenna changeover, that is, when the RSSI after the antenna changeover is rather degraded, connection of the antenna for which the RSSI is lower sometimes remains for a long time, thus degrading the receiving performance.
The present invention contemplates elimination of the above problems and it is an object of the present invention to provide a diversity receiver which can obtain, with simplified construction, good communication quality regardless of the fading speed.
To accomplish the above object, a diversity receiver according to a first aspect of the invention comprises an antenna changeover switch for connecting one of a plurality of antennas to deliver a receiving signal, a receiving unit connected to the antenna changeover switch to demodulate the receiving signal sent from the antenna changeover switch and detect signal intensity of the receiving signal, and antenna selecting means for comparing the receiving signal intensity detected by the receiving unit with a changeover threshold value so as to control the antenna changeover switch, wherein the antenna selecting means controls the antenna changeover switch such that when the receiving signal intensity falls below the changeover threshold value under a state that one antenna is selected, the antenna selecting means causes the antenna changeover switch to transfer from the one antenna to a second antenna and when, at the termination of a first lapse time from the first transfer, the receiving signal intensity is below that immediately before the first transfer, the antenna selecting means again causes the antenna changeover switch to transfer from the second antenna to the one antenna, the antenna selecting means being prevented from causing the antenna changeover switch to transfer until a second lapse time from the second transfer expires.
With this construction, when the receiving signal intensity (RSSI) rather decreases after the first transfer, the antenna changeover is again carried out and then, the changeover is not effected during the second lapse time, thereby ensuring that frequent changeover can be prevented and besides a longtime connection, to the receiving unit, of an antenna for which the receiving signal intensity is low (or a longtime transfer of the antenna changeover switch to an antenna for which the receiving signal intensity is low) can be avoided. Accordingly, the diversity receiver can be provided which can obtain, with the simplified construction, good communication quality regardless of the fading speed.
A diversity receiver according to a second aspect of the invention comprises three or more antennas, an antenna changeover switch for connecting one of the antennas to deliver a receiving signal, a receiving unit including a circuit connected to the antenna changeover switch to demodulate the receiving signal from the antenna changeover switch so as to convert it into a baseband signal and detect intensity of the receiving signal, and antenna selecting means for comparing the receiving signal intensity detected by the receiving unit with a changeover threshold value so as to control the antenna changeover switch, wherein the antenna selecting means controls the antenna changeover switch such that when the receiving signal intensity falls below the changeover threshold value, the antenna selecting means causes the antenna changeover switch to perform a first transfer operation in which the antenna change over switch transfers from one antenna to a second antenna and when, at the termination of a first lapse time from the first transfer operation, the receiving signal intensity is below that immediately before the first transfer operation, the antenna selecting means causes the antenna changeover switch to perform a second transfer operation in which the antenna changeover switch sequentially transfers to the remaining antennas, the antenna selecting means being prevented from causing the antenna changeover switch to perform the first transfer operation until a second lapse time from the second transfer operation expires, and the second lapse time is longer than the first lapse time.
With this construction, when the receiving signal intensity (RSSI) rather decreases after the first transfer operation, the antenna changeover switch sequentially transfers to the remaining antennas so as to sequentially retrieve an antenna for which the receiving signal intensity exceeds that immediately before the first transfer operation and connect the retrieved antenna to the receiving unit and after the connection is established, the antenna changeover switch is prevented from transferring until the second lapse time expires, so that even when the number of diversity branches, that is, the number of antennas increases to three or more in order to increase the diversity gain, frequent transfers or changeovers can be prevented and a longtime connection of an antenna for which the receiving signal intensity is low can be avoided.
Alternatively, the second lapse time is not constant but is changed suitably. With this construction, the time interval for antenna changeover becomes non-uniform and an uncomfortable feeling due to changeover noise given to audibility can be decreased.
Preferably, the second lapse time is set by adding pseudo-random numbers to the first lapse time. With this construction, the time interval for antenna changeover becomes irregular (random) and an uncomfortable feeling due to changeover noise given to audibility can be lowered.